Porcine epidemic diarrhea (PED) virus was first detected in the United States in May 2013 in several apparently unrelated farms, and has spread rapidly to many other swine herds creating a nationwide epidemic. This situation raises two main questions that are still being researched: 1) where did this virus come from? and 2) how is it spreading between herds?
To start answering the first question, where did it come from, we have to ask ourselves if this was really a new virus or if it was already in the United States and had gone undetected. Both diagnostic laboratories at Iowa State University and the University of Minnesota did testing on banked samples from 2012 and 2013 and could only detect PED virus after mid April 2013.
Therefore, we can be reasonably confident that the virus was not in the U.S. swine population before that time. PED virus has been described in Europe and in Asia in the past. No major outbreaks of this disease have been described in recent years in Europe.
In contrast, PED virus outbreaks affecting numerous farms and causing large mortalities have been described recently in important swine-producing Asian countries such as China, South Korea, Japan and Thailand.
If we compare the genetic material of the PED virus strain detected in the United States with other publicly available PED virus strains, the closest matches are strains from China. All this evidence is suggesting that PED virus was introduced in the U.S. swine population in the spring of 2013 and the most likely origin is China.
Assuming that the virus came from China, how did it get here? The United States does not import pigs from China so there must be a different way. The U.S. Department of Agriculture (USDA), in collaboration with the University of Minnesota, performed an epidemiological investigation on the earliest cases of PED virus in the United States to assess potential risk factors that could explain the introduction of the virus into the country. Unfortunately, this study could not come up with a clear answer. Early investigations also looked into the unlikely possibility of the virus being imported in vitamin or mineral mixes being used in swine feeds. However, none of the feed ingredients tested had infectious PED virus.
Another possibility is that PED virus came with people. PED virus has not been reported to infect people, however, people can act as mechanical carriers. It seems an unlikely scenario, given the good biosecurity practices generally adopted in the U.S. swine industry. However, because of the booming situation of the Chinese swine industry, there are many swine professionals (veterinarians, producers, builders, company representatives) who frequently travel back and forth between the United States and China and visit farms in both countries.
Therefore, we cannot rule out people as a source of entry. This stresses the importance of biosecurity practices. Enforcing strict visitor rules can prevent disease introductions in your herd. Keeping track of visitors records (who enters your herd and where have they been before) will help with the investigation of the origin of an outbreak if it were to happen.
Secondly, how is PED virus spreading from farm to farm in the United States? There is a bulk of research being performed on this issue as we speak. We will tackle this question in the next column once the results of that research are fully available.
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